The field of the invention is support structures for supporting electromagnetic coils which are subjected to high axial and radial forces.
The use of high field magnets, in applications such as . magnetic resonance imaging in medical diagnosis, is well known. Such high field magnets are typically made with several discrete electromagnetic coils which are held coaxially with respect to each other by a common frame. The coils are typically superconducting, that is, they are immersed in a cryostat which contains liquid helium to cool the magnet coils to approximately 4.2.degree. Kelvin. At that temperature, the coils have zero resistance and are capable of conducting a very high current density and producing magnetic field strengths of 1.5 Tesla or more. At such field strengths, considerable stress is placed on the magnet coil support structure. Adjacent coils have mutual axial attractive forces of 100,000 to 200,000 lbs. Self repulsion within a coil produces radial or "hoop stresses" of 200,000-500,000 lbs.
The magnet support structure must resist these magnetic field related stresses, and provide the rigidity necessary to ensure that the magnetic field remains uniform over time. This latter requirement was thought most recently to require that the magnet support structure be relatively massive and inflexible.
Prior support structure designs have been of a solid walled. barrel shape. Such structures have been difficult and expensive to manufacture because of the size and weight of the structures which have been used to make up the barrel and because of the difficulty of machining such large elements. Also, the weight of such designs increased the cost of ancillary magnet support structure and limited the use of such magnets to areas where the floor would support high loads. The closed construction of such designs also made assembly more difficult and obstructed the placement of components inside the cryostat.
Moreover, the rigidity of such magnet support structures may have contributed to slippage of the magnet coils relative to the support structure when the axial force to which the coils were subjected changed, for example, when the magnet was energized, or when there were minor misalignments between the coils.
Also, the construction of the barrel shaped support 10 structure resulted in a high mutual inductance between the coils and the support structure by providing a path for eddy currents around the support structure's circumference and along its length. With such a high mutual inductance, changes in the position of the coil with respect to the support structure may have resulted in surges in the coil current. Such surges can trigger a "quench" of the superconducting magnet in which the magnet reverts to a non-persistent or non-superconducting state.